那个条件是什么意思？

Question

What should this current -> yes evaluate in if condition? (It's not complete code you can assume any question in yes)

typedef struct node {
    char *question;
    struct node *no;
    struct node *yes;
} node;

node *current;

if (current->yes) {
    current = current->yes;
}

Thanks a lot for helping

Answer 1

这意味着如果current->yes不是0 （不是空指针）。

Answer 2

You're getting the address of yes member of type struct node * , by dereferencing current object. Which is, if allocated the memory properly, it'll be non zero, and if not it'll be 0 or NULL . For example memory allocations can fail.

So, basically the condition: if (current->yes) { ... } checks whether memory is allocated properly or not.

Answer 3

The following code :

if (current->yes) {
    current = current->yes;
}

is equivalent to

if (current->yes != NULL) {
    current = current->yes;
}

So the condition is checking yes to not be NULL pointer

Answer 4

The if() construct in C is defined to evaluate its argument and compare it to 0 . As such, if(foo) is always equivalent to if(foo != 0) or if(foo != NULL) , considering that NULL compares equal to 0 .

Or, put another way, if(myPointer) checks whether the pointer points to a valid object. This is a very common idiom in C programming.

---

This is true even if you write if(a == b) . This is fully equivalent to if((a == b) != 0) : The value of a == b is either 0 for false or 1 for true, and that result is then compared to 0 by the abstract machine to determine which branch to execute. Obviously, compilers will directly use the result of the comparison in if(a == b) , because that's equivalent to producing the 1 or 0 and then comparing a second time to 0 . However, the language is defined with the extra comparison against 0 to make it fully agnostic about what kind of expression is used within the parentheses. All the language cares about is that the resulting value is comparable to 0 .